Glass composition for crystallized glass

ABSTRACT

A polished glass disk medium substrate suitable for use as a substrate for a hard disk, a hard disk containing the substrate and methods for making the substrate. The substrate containing glass forming raw materials may be formed so as to have a Young&#39;s modulus of 110 or higher.

RELATED APPLICATION

[0001] This application claims priority to Japanese Patent ApplicationNo. 2000-100855 filed in Japan on Apr. 3, 2000, the contents of whichare hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a glass composition, andspecifically relates to a glass composition suited for crystallizedglass. More specifically, the present invention relates to a compositionfor crystallized glass disk medium. Such disk medium include hard disks,magnetic disks, optical disks and magnetic-optical disks

DESCRIPTION OF THE PRIOR ART

[0003] Aluminum and glass are known materials suitable for use asmagnetic disk substrates. Among these substrates, glass substrates havebeen the focus of most attention due to their superior surfacesmoothness and mechanical strength. Such glass substrates includechemically reinforced glass substrates strengthened by ion exchange onthe surface, and crystallized glass substrates having strengthened bondsby depositing a crystal component on the substrate.

[0004] The performance demands of recent substrates have become moresevere day by day, and improved performance is particularly soughtregarding strength, flex and warp during high-speed rotation. This typeof performance can be expressed by the Young's modulus of the substratematerial, in which a higher numerical value is desirable.

[0005] For example, the composition disclosed in Japanese Laid-OpenPatent Application No. 11-322362 attains a Young's modulus value of 130or greater. However, this prior art requires extremely high thermalprocessing temperatures which complicate the manufacturing process, thatis, this art requires a primary processing temperature of 800° C., and asecondary processing temperature of 1,000° C.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to provide an improvedglass composition.

[0007] Another object of the present invention is to provide a glasscomposition having a high Young's modulus and which is highly suited formass production.

[0008] These objects are attained with a glass composition of thepresent invention desirably having the main components within the rangesdescribed below:

[0009] about 35 wt % or more, but less than about 50 wt % SiO₂;

[0010] about 5 wt % or more, but less than about 20 wt % Al₂O₃;

[0011] about 9 wt % or more, but less than about 25 wt % MgO;

[0012] about 0.1 wt % or more, but less than about 12 wt % TiO₂; and

[0013] about 5 wt % or more, but less than about 10 wt % ZnO.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] The preferred embodiments of the present invention are describedhereinafter.

[0015] These objects are attained with a glass composition of thepresent invention desirably having the main components within the rangesdescribed below:

[0016] about 35 wt % or more, but less than about 50 wt % SiO₂;

[0017] about 5 wt % or more, but less than about 20 wt % Al₂O₃;

[0018] about 9 wt % or more, but less than about 25 wt % MgO;

[0019] about 0.1 wt % or more, but less than about 12 wt % TiO₂; and

[0020] about 5 wt % or more, but less than about 10 wt % ZnO.

[0021] When the composition content of SiO₂ used as a glass formingoxide is less than about 35 wt %, melting characteristics are typicallyadversely affected, and when the percentage exceeds about 50 wt %, astabilized state of glass is achieved and crystal deposition typicallybecomes difficult.

[0022] Aluminum oxide (Al₂O₃) is an intermediate oxide of glass, and isa structural component of the crystal-phase magnesium-aluminum crystalsformed during heating. When the composition content is less than about 5wt %, there are typically few crystals formed, and the desired strengthis not obtained, whereas when the percentage exceeds about 20 wt %, themelting temperature is typically raised and devitrification readilyoccurs.

[0023] Magnesium oxide (MgO) is a fluxing agent, which is added toinduce the crystal particles to nucleate and form crystal particleclusters. When the composition content is less than about 9 wt %, theworking temperature range is typically narrowed, and the chemicaldurability of the glass matrix phase is not typically improved. When thecomposition content exceeds about 25 wt %, other crystal phase matter isoften deposited and the desired strength is typically difficult toobtain.

[0024] Titanium oxide (TiO₂) is a crystal nucleating agent, which isoften an essential component for magnesium silicate crystal deposition.Furthermore, TiO₂ functions as a fluxing agent to improve stabilityduring production. When the composition content is less than about 0.1wt %, melting characteristics are typically adversely affected, andcrystal growth is often difficult. When the content exceeds about 12 wt%, crystallization typically progresses rapidly, the crystallizationstate often becomes difficult to control, the deposited crystals aretypically coarse with heterogeneity of the crystal phase, and a finehomogeneous crystal structure often cannot be obtained, such that therequired surface smoothness for use as a disk substrate is difficult toobtain by a polishing process. Furthermore, devitrification readilyoccurs during fusion molding, and mass production characteristics arereduced.

[0025] Zinc oxide (ZnO) functions as a fluxing agent which augmentsuniform crystal deposition. When the composition content is less thanabout 5 wt %, there is typically insufficient improvement in crystalhomogeneity. When the composition content exceeds about 10 wt %, theglass becomes stable, and crystallization is suppressed, such that thedesired strength is often difficult to obtain.

[0026] The manufacturing method is described below. The raw materials ofthe ultimately produced glass substrate are thoroughly mixed in specificproportions, then introduced to a platinum crucible and melted. Aftermelting, the melted material is poured into a mold to form anapproximate shape. Then the material is annealed to room temperature.Next, the material is maintained at a primary heating processtemperature of about 500 to about 680° C. during a primary process(heating process) to generate crystal nuclei. Then, the material ismaintained at a secondary heating process temperature of about 680 toabout 800° C. during a secondary process to grow crystal nuclei. Thenthe material is cooled to obtain the crystallized glass.

[0027] This material may be used as a disk substrate by processing suchas polishing to attain a desired shape and thickness.

[0028] By using the above raw materials and the process describedherein, an extremely high Young's modulus and high mass productioncharacteristics are obtainable. Even higher performance is obtained byadding the components described below in a suitable range.

[0029] Stability during manufacture is improved by the addition of Li₂O,which functions as a fluxing agent. When the composition content is lessthan about 0.1 wt %, there is inadequate improvement in meltingcharacteristics. When the composition content exceeds about 8 wt %,stability often decreases during the polishing and washing processes.

[0030] Phosphoric anhydride (P₂O₅), which functions as a fluxing agent,is a nucleating agent for depositing silicate crystals, and is animportant component for uniform deposition of crystals on the entiretyof the glass. When the composition content is less than about 0.1 wt %,sufficient formation of crystal nuclei typically becomes difficult,crystal particles are often coarse, heterogeneous crystal depositionoften occurs, the desired fine homogeneous crystal structure may bedifficult to obtain, such that the required surface smoothness for useas a disk substrate may be difficult to obtain by a polishing process.When the content exceeds about 5.0 wt %, reactivity to the filter mediumincreases during melting, and devitrification increases so as to reducemass production characteristics during fusion molding. Chemicaldurability typically decreases, there is concern that the magnetic layermay be affected, and stability is often reduced during the polishing andwashing processes.

[0031] Adding ZrO₂ which functions as a glass modifying oxidant alsofunctions effectively as a glass crystal nucleating agent. When thecontent ratio is less than about 0.1 wt %, sufficient formation ofcrystal nuclei typically becomes difficult, crystal particles are oftencoarse, heterogeneous crystal deposition often occurs, the desired finehomogeneous crystal structure may be difficult to obtain, such that therequired surface smoothness for use as a disk substrate may be difficultto obtain by a polishing process. Furthermore, chemical durability andmigration resistance are often reduced, there is concern that themagnetic layer may be affected, and stability is often reduced duringthe polishing and washing processes. When the content exceeds about 12wt %, the melting temperature is raised, devitrification readily occurs,and fusion molding typically becomes difficult. Furthermore, thedeposition crystal phase fluctuates such that desired characteristicsare often difficult to obtain.

[0032] The addition of CaO, which functions as a fluxing agent,supplements uniform crystal deposition. When the composition content isless than about 0.1 wt %, sufficient improvement in crystal homogeneityis not typically obtained. When the content exceeds about 9 wt %,chemical durability is not typically improved.

[0033] Crystal nucleating material is increased by the addition ofNb₂O₅, which works as a fluxing agent. When the composition content isless than about 0.1 wt %, there is often inadequate rigidityimprovement. When the composition content exceeds about 9 wt %,crystallization of the glass typically becomes unstable, the depositioncrystal phase typically becomes uncontrollable, and the desiredcharacteristics are often difficult to obtain.

[0034] The addition of Ta₂O₅, which works as a fluxing agent, improvesfusion and strength, and also improves chemical durability in the glassmatrix phase. When the composition content is less than about 0.1 wt %,there is typically inadequate rigidity improvement. When the compositioncontent exceeds about 9 wt %, crystallization of the glass typicallybecomes unstable, the deposition crystal phase becomes uncontrollable,and the desired characteristics are often difficult to obtain.

[0035] Stability during manufacture is improved by the addition of K₂O,which functions as a fluxing agent. When the composition content is lessthan about 0.1 wt %, there is inadequate improvement in meltingcharacteristics. When the composition content exceeds about 9 wt %, theglass typically becomes stable and crystallization is suppressed,chemical durability is often reduced, and there is concern that themagnetic layer will be affected, and stability often decreases duringthe polishing and washing processes.

[0036] Glass phase splitting is promoted by adding B₂O₃, which works asa former, and accelerates crystal deposition and growth. When thecomposition content is less than about 0.1 wt %, improvement of meltingcharacteristics is typically inadequate. When the composition contentexceeds about 9 wt %, glass devitrification readily occurs, moldingtypically becomes difficult, and the crystals often become coarse suchthat fine crystals is difficult to obtain.

[0037] Rigidity is improved by adding Y₂O₃, which functions as a fluxingagent. When the composition content is less than about 0.1 wt %, thereis typically inadequate rigidity improvement. When the compositioncontent exceeds about 9 wt %, crystal deposition is often suppressed,sufficient crystallization is difficult to obtain, and desiredcharacteristics are often not attained.

[0038] Stability during mass production is improved by adding Sb₂O₃,which functions as a clarifier. When the composition content is lessthan about 0.1 wt %, there is typically insufficient clarificationeffect, and production characteristics are typically reduced. When thecomposition content exceeds about 9 wt %, crystallization of the glassoften becomes unstable, the deposition crystal phase typically becomesuncontrollable, and the desired characteristics are often difficult toobtain.

[0039] Stability during production is improved by adding As₂O₃, whichfunctions as a clarifier. When the composition content is less thanabout 0.1 wt %, there is often insufficient clarification effect, andproduction characteristics are often reduced. When the compositioncontent exceeds about 9 wt %, crystallization of the glass typicallybecomes unstable, the deposition crystal phase typically becomesuncontrollable, and the desired characteristics are often difficult toobtain.

[0040] The glasses of the present invention may have one or morecrystalline phases and an amorphous phase. The crystalline phasesrepresent about 50 to about 60 percent of the total glass composition.Preferred embodiments include a main crystalline phase of clinoenstatitewhich desirably represents at least about 80 percent by weight of thetotal of all crystalline phases. Preferred embodiments may also includea secondary crystalline phase of, for example, enstatite magnesiumaluminum silicate, and/or zinc titanium oxide which desirably representsless than about 20 percent by weight of the total crystalline phase.

[0041] Although the present invention is described in detail in thefollowing examples, the invention is not limited to these examples.Tables 1-4 show the glass composition in percent-by-weight of Examples1-34. Glass substrates were obtained by the previously describedmanufacturing method in accordance with these numerical examples.

[0042] In the tables, C1 represents a crystal phase of clinoenstatite(MgSiO₃), C2 represents a crystal phase of enstatite (MgSiO₃), M1represents a crystal phase of magnesium aluminum silicate {(Mg Al)SiO₃},Z1 represents a crystal phase of zinc titanium oxide (Zn₂Ti₃O₈) and Z2represents a crystal phase of zinc titanium oxide (Zn₂TiO₄). TABLE 1 Ex.01 Ex. 02 Ex. 03 Ex. 04 Ex. 05 Ex. 06 Ex. 07 Ex. 08 Ex. 09 Ex. 10 SiO₂45.0 44.0 45.0 45.0 45.0 42.0 41.0 40.0 37.0 40.0 Al₂O₃ 20.0 20.0 18.520.0 20.0 20.0 20.0 20.0 20.0 15.0 MgO 17.0 17.0 17.0 15.0 17.0 17.017.0 17.0 17.0 13.0 TiO₂ 13.0 13.0 13.0 13.0 10.5 13.0 13.0 13.0 13.013.0 Li₂O 8.0 13.0 ZnO 5.0 6.0 6.5 7.0 7.5 8.0 9.0 10.0 5.0 6.0 PrimaryTreatment Temperature (° C.) 660 660 660 660 660 660 660 660 660 660Secondary Treatment Temperature (° C.) 700 700 700 700 700 700 700 700700 700 Primary Treatment Time (hr) 5 5 5 5 5 5 5 5 5 5 SecondaryTreatment Temperature (hr) 5 5 5 5 5 5 5 5 5 5 Young's Modules (G Pa)150.2 151.2 150.1 149.7 149.3 153.5 154.8 156.1 154.3 145.4 Diameter ofCrystal (nm) 30 30 30 30 30 30 30 30 30 30 Main Crystal Phase C1 C1 C1C1 C1 C1 C1 C1 C1 C1 Secondary Crystal Phase C2 C2 C2 C2 C2 C2 C2 C2 C2C2 Other Crystal Phase M1 M1 M1 M1 M1 M1 M1 M1 M1 M1 Other Crystal PhaseZ1 Z1 Z1 Z1 Z1 Z1 Z1 Z1 Z1 Z1 Other Crystal Phase Z2 Z2 Z2 Z2 Z2 Z2 Z2Z2 Z2 Z2

[0043] TABLE 2 Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Ex. 18Ex. 19 Ex. 20 SiO₂ 43.0 40.0 40.5 40.0 40.0 40.0 42.0 40.0 40.0 38.0Al₂O₃ 20.0 20.0 20.0 18.0 20.0 18.0 21.0 20.0 21.5 20.0 MgO 17.0 17.017.0 15.0 17.0 15.0 17.0 17.0 17.0 17.0 TiO₂ 13.0 13.0 13.0 13.0 12.011.0 13.0 12.0 13.0 13.0 ZnO 6.5 7.0 7.5 8.0 9.0 10.0 5.0 6.0 6.5 7.0P₂O₅ 0.5 3.0 ZrO₂ 2.0 6.0 CaO 2.0 6.0 Nb₂O₅ 2.0 5.0 Ta₂O₅ 2.0 5.0Primary Treatment Temperature (° C.) 660 660 660 660 660 660 660 660 660660 Secondary Treatment Temperature (° C.) 700 700 700 700 700 700 700700 700 700 Primary Treatment Time (hr) 5 5 5 5 5 5 5 5 5 5 SecondaryTreatment Temperature (hr) 5 5 5 5 5 5 5 5 5 5 Young's Modules (G Pa)152.2 154.8 154.6 152.9 154.8 152.1 152.7 153.5 155.3 156.6 Diameter ofCrystal (nm) 30 30 30 30 30 30 30 30 30 30 Main Crystal Phase C1 C1 C1C1 C1 C1 C1 C1 C1 C1 Secondary Crystal Phase C2 C2 C2 C2 C2 C2 C2 C2 C2C2 Other Crystal Phase M1 M1 M1 M1 M1 M1 M1 M1 M1 M1 Other Crystal PhaseZ1 Z1 Z1 Z1 Z1 Z1 Z1 Z1 Z1 Z1 Other Crystal Phase Z2 Z2 Z2 Z2 Z2 Z2 Z2Z2 Z2 Z2

[0044] TABLE 3 Ex. 21 Ex. 22 Ex. 23 Ex. 24 Ex. 25 Ex. 26 Ex. 27 Ex. 28Ex. 29 Ex. 30 SiO₂ 40.0 40.0 40.0 40.0 42.0 40.0 40.0 42.5 40.5 40.0Al₂O₃ 22.3 20.0 20.0 14.0 20.0 20.0 23.3 20.0 20.0 18.0 MgO 17.0 15.017.0 17.0 18.0 17.0 17.0 17.0 17.0 17.0 TiO₂ 13.0 13.0 12.0 13.0 13.013.0 13.0 13.0 13.0 13.0 ZnO 7.5 8.0 9.0 10.0 5.0 6.0 6.5 7.0 7.5 8.0K₂O 0.2 4.0 B₂O₃ 2.0 6.0 Y₂O₃ 2.0 4.0 Sb₂O₃ 0.2 0.5 2.0 4.0 PrimaryTreatment Temperature (° C.) 660 660 660 660 660 660 660 660 660 660Secondary Treatment Temperature (° C.) 700 700 700 700 700 700 700 700700 700 Primary Treatment Time (hr) 5 5 5 5 5 5 5 5 5 5 SecondaryTreatment Temperature (hr) 5 5 5 5 5 5 5 5 5 5 Young's Modules (G Pa)156.1 153.8 154.8 153.3 153 154.4 156.1 152.7 154.6 154.3 Diameter ofCrystal (nm) 30 30 30 30 30 30 30 30 30 30 Main Crystal Phase C1 C1 C1C1 C1 C1 C1 C1 C1 C1 Secondary Crystal Phase C2 C2 C2 C2 C2 C2 C2 C2 C2C2 Other Crystal Phase M1 M1 M1 M1 M1 M1 M1 M1 M1 M1 Other Crystal PhaseZ1 Z1 Z1 Z1 Z1 Z1 Z1 Z1 Z1 Z1 Other Crystal Phase Z2 Z2 Z2 Z2 Z2 Z2 Z2Z2 Z2 Z2

[0045] TABLE 4 Ex. 31 Ex. 32 Ex. 33 Ex. 34 SiO₂ 40.0 40.0 43.0 40.0Al₂O₃ 16.0 12.0 20.0 19.0 MgO 17.0 17.0 17.0 17.0 TiO₂ 13.0 13.0 13.013.0 ZnO 9.0 10.0 5.0 6.0 Sb₂O₃ 5.0 8.0 As₂O₃ 2.0 5.0 Primary Treatment660 660 660 660 Temperature (° C.) Secondary Treatment 700 700 700 700Temperature (° C.) Primary Treatment Time (hr) 5 5 5 5 SecondaryTreatment 5 5 5 5 Temperature (hr) Young's Modules (G Pa) 153.8 152.4151.6 153.9 Diameter of Crystal (nm) 30 30 30 30 Main Crystal Phase C1C1 C1 C1 Secondary Crystal Phase C2 C2 C2 C2 Other Crystal Phase M1 M1M1 M1 Other Crystal Phase Z1 Z1 Z1 Z1 Other Crystal Phase Z2 Z2 Z2 Z2

[0046] The present invention provides a glass substrate having excellentproduction characteristics and a Young's modulus of 110 or higher.

[0047] Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modification will be apparent to those skilledin the art. Therefore, unless such changes and modifications depart fromthe scope of the present invention, they should be construed as beingincluded therein.

What is claimed is:
 1. A polished glass disk medium substrate formed ofa mixture of glass forming raw materials comprising about 35 wt % ormore, but less than about 50 wt % SiO₂; about 5 wt % or more, but lessthan about 20 wt % Al₂O₃; about 9 wt % or more, but less than about 25wt % MgO; about 0.1 w % or more, but less than about 12w % TiO₂; andabout 5 w % or more, but less than about 10 w % ZnO.
 2. The polishedglass disk medium substrate according to claim 1 , wherein the rawmaterials further comprise about 0.1% to about 8% by weight Li₂O.
 3. Thepolished glass disk medium substrate according to claim 1 , wherein theraw materials further comprise about 0.1% to about 5% by weight P₂O₅. 4.The polished glass disk medium substrate according to claim 1 , whereinthe raw materials further comprise about 0.1% to about 12% by weightZrO₂.
 5. The polished glass disk medium substrate according to claim 1 ,wherein the raw materials further comprise about 0.1% to about 9% byweight CaO.
 6. The polished glass disk medium substrate according toclaim 1 , wherein the raw materials further comprise about 0.1% to about9% by weight Nb₂O₅.
 7. The polished glass disk medium substrateaccording to claim 1 , wherein the raw materials further comprise about0.1% to about 9% by weight Ta₂O₅.
 8. The polished glass disk mediumsubstrate according to claim 1 , wherein the raw materials furthercomprise about 0.1% to about 9% by weight K₂O.
 9. The polished glassdisk medium substrate according to claim 1 , wherein the raw materialsfurther comprise about 0.1% to about 9% by weight B₂O₃.
 10. The polishedglass disk medium substrate according to claim 1 , wherein the rawmaterials further comprise about 0.1% to about 9% by weight Y₂O₃. 11.The polished glass disk medium substrate according to claim 1 , whereinthe raw materials further comprise about 0.1% to about 9% by weightSb₂O₃.
 12. The polished glass disk medium substrate according to claim 1, wherein the raw materials further comprise about 0.1% to about 9% byweight As₂O₃.
 13. The polished glass disk medium substrate according toclaim 1 , said raw materials consisting essentially of about 35% toabout 50% by weight SiO₂; about 5% to about 20% by weight Al₂O₃; about9% to about 25% by weight MgO; about 0.1% to about 12% by weight TiO₂;and about 5% to about 1% by weight ZnO.
 14. The polished glass diskmedium substrate according to claim 13 , further containing one or moreof the following: about 0.1% to about 8% by weight Li₂O; about 0.1% toabout 5% by weight P₂O₅; about 0.1% to about 12% by weight ZrO₂; about0.1% to about 9% by weight CaO; about 0.1% to about 9% by weight Nb₂O₅;about 0.1% to about 9% by weight Ta₂O₅; about 0.1% to about 9% by weightK₂O; about 0.1% to about 9% by weight B₂O₃; about 0.1% to about 9% byweight Y₂O₃; about 0.1% to about 9% by weight Sb₂O₃; and about 0.1% toabout 9% by weight As₂O₃.
 15. The polished glass disk medium substrateaccording to claim 13 , wherein said substrate is essentially free ofBaO, ZrO₂, B₂O₃ and NiO.
 16. The polished glass disk medium substrateaccording to claim 1 , comprising crystalline phases and amorphousphases.
 17. The polished glass disk medium substrate according to claim16 , wherein the crystalline phases represent about 50 to about 60percent by weight of the total glass composition.
 18. The polished glassdisk medium substrate according to claim 16 , comprising a crystallinephase of clinoenstatite.
 19. The polished glass disk medium substrateaccording to claim 18 , wherein the crystalline phase of clinoenstatiterepresents at least about 80 percent by weight of the crystallinephases.
 20. The polished glass disk medium substrate according to claim16 , comprising a crystalline phase of enstatite.
 21. The polished glassdisk medium substrate according to claim 20 , wherein the crystallinephase of enstatite represents less than or equal to about 20 percent byweight of the crystalline phases.
 22. The polished glass disk mediumsubstrate according to claim 16 , comprising a crystalline phase ofmagnesium aluminum silicate.
 23. The polished glass disk mediumsubstrate according to claim 22 , wherein the crystaline phase ofmagnesium aluminum silicate represents less than or equal to about 20percent by weight of the crystalline phases.
 24. The polished glass diskmedium substrate according to claim 16 , comprising a crystalline phaseof Zn₂Ti₃O₈.
 25. The polished glass disk medium substrate according toclaim 24 , wherein the crystalline phase of Zn₂Ti₃O₈ represents lessthan or equal to about 20 percent by weight of the crystalline phases.26. The polished glass disk medium substrate according to claim 16 ,comprising a crystalline phase of Zn₂TiO₄.
 27. The polished glass diskmedium substrate according to claim 26 , wherein the crystalline phaseof Zn₂TiO₄ represents less than or equal to about 20 percent by weightof the crystalline phase.
 28. The polished glass disk medium substrateaccording to claim 1 , comprising a main crystalline phase ofclinoenstatite and a secondary crystalline phase of enstatite.
 29. Thepolished glass disk medium substrate according to claim 28 , furthercomprising one or more of a crystalline phase of magnesium aluminumsilicate, a crystalline phase of zinc titanium oxide, and a crystallinephase of zinc titanium oxide.
 30. The polished glass disk mediumsubstrate according to claim 1 , wherein said glass substrate has aYoung's modulus of 110 or higher.
 31. The polished glass disk mediumsubstrate according to claim 1 , wherein said substrate is prepared byheating glass forming raw materials to a temperature, T₁, between about500 and 680° C. to generate crystal nuclei; heating at a temperature,T₂, between about 680 and abot 800° C. to grow crystal nuclei; andcooling to obtain crystallized glass.
 32. A recording disk comprisingthe polished glass disk medium substrate defined in claim 1 .
 33. Therecording disk according to claim 32 , wherein said recording disk is ahard disk.
 34. The recording disk according to claim 32 , wherein saidrecording disk is a magnetic disk.
 35. The recording disk according toclaim 32 , wherein said recording disk is an optical disk.
 36. Therecording disk according to claim 32 , wherein said recording disk is amagnetic-optical disk.
 37. A method of making a glass disk mediumsubstrate comprising: heating glass forming raw materials to atemperature sufficiently high to melt the raw materials; forming a diskmedium substrate; and crystallizing the disk medium substrate, whereinsaid crystallizing comprises heating the disk medium substrate to atemperature, T₁, between about 500 and 680° C. to generate crystalnuclei; heating at a temperature, T₂, between about 680 and about 800°C. to grow crystal nuclei; and cooling to obtain crystalized glass. 38.The method according to claim 37 , further comprising polishing saidglass disk medium substrate.
 39. The method according to claim 37 ,wherein said glass disk medium substrate formed has a Young's modulus of110 or higher.